This invention relates to electrochemical apparatus and more particularly to the improved polarographic type electrochemical cells for measuring the concentration of electrochemically active species in fluids. The invention is particularly concerned with a method for determining the degree of fouling of the membrane covering used on those electrochemical cells which operate in a manner to avoid the depletion of the species being measured in the sample in which the cell is immersed. Thus, the invention relates to a method for detecting the degree of membrane fouling in electrochemical cells of the type disclosed in U.S. Pat. No. 3,260,656 issued to J. W. Ross, Jr. on Jan. 12, 1966, and No. 4,076,596 issued to J. G. Connery, et al. on Feb. 28, 1978.
The description of the electrochemical cells of the type for which this method is useful is disclosed in U.S. Pat. No. 4,076,596 and is hereby incorporated by reference.
As set forth in the last mentioned U.S. patent, the basic polarographic apparatus as it was improved by Clark in a manner described in U.S. Pat. No. 2,913,386 is known as the Clark cell. The Clark cell utilizes a dual electrode structure immersed in an electrolyte and encased in a membrane which is permeable to the species to be measured, for instance, gaseous oxygen. Typically, when used for oxygen analysis the cathode in the Clark cell is formed of platinum or gold and is located closely adjacent to the membrane while the anode is formed of silver or lead, with an electrolyte usually made of an aqueous alkaline halide solution. In operation, the Clark cell is characterized by the fact that the cell consumes the species being measured and therefore causes a depletion of the species from the fluid sample in which the cell is immersed.
To avoid the disadvantages such as stirring dependencies, etc. which result from such a depletion of the sample, a cell structure of the type shown in the Ross U.S. Pat. No. 3,260,656 was proposed. That cell utilizes an electrode system which consumes the species being measured at one electrode and which generates a like quantity of the species at the electrode of opposite polarity, with those electrodes being closely spaced so as to avoid depletion of the species from the sample. The Ross type was further improved by the Connery et al cell structure of U.S. Pat. No. 4,076,596. For the purpose of this description, those cells which characteristically deplete the sample of the species being measured are referred to as Clark-type cells, or cells which operate in the Clark mode, whereas those cells such as the Ross or Connery et al cells are referred to generically as the Ross-type cells or cells operating in the Ross mode, in that they operate without depleting sample of the species being measured.
It will be evident that cells operating in the Clark mode will have current outputs which reduce as fouling of the membrane covering of the cell progresses, since the species being consumed by the cell is less able to pass throught the membrane whereas Ross-type cells have current outputs which will not change as a result of the fouling of the membrane since there is no requirement for the species to pass through the membrane to maintain a particular current output. Thus, as fouling progresses on the membrane of a Ross-type cell, the result is a decrease in the rate of response of the cell to changes in concentration of the species in the sample and observation of the output current of the cell provides no indication as to whether or not fouling is progressing. It is therefore an object of this invention to provide a method for determining the degree of fouling in a Ross-type cell.